Multi-layer phase change memory device

ABSTRACT

A phase change memory (PCM) cell comprises a first electrode comprised of a first electrically conductive material, a second electrode comprised of a second electrically conductive material, a first phase change layer positioned between the first electrode and the second electrode and being comprised of a first phase change material, and a second phase change layer positioned between the first electrode and the second electrode and being comprised of a second phase change material. The first phase change material has a first resistivity, the second phase change material has a second resistivity, and wherein the first resistivity is at least two times the second resistivity.

BACKGROUND

The present invention relates to computer memory, and more specifically,to phase change material memory devices with multiple layers.

Phase change memory (PCM) can be utilized for both training andinference in analog computing for artificial intelligence. The phasechange memory structures can include phase change memristive deviceswith tunable conductivities from device to device and overall highdevice resistance with high retention to minimize energy consumption.Mixing (a.k.a. doping) PCM materials with dielectrics and poorlyelectrically conductive materials such as, for example, silicon dioxide(SiO2), silicon monoxide (SiO), silicon oxynitride (SiON), siliconoxycarbide (SiOC), and aluminum nitride (AlN), can increase bothcrystallization temperature and resistivity, providing improvedresistance state retention and low programming power.

SUMMARY

According to an embodiment of the present disclosure, a phase changememory (PCM) cell comprises a first electrode comprised of a firstelectrically conductive material, a second electrode comprised of asecond electrically conductive material, a first phase change layerpositioned between the first electrode and the second electrode andbeing comprised of a first phase change material, and a second phasechange layer positioned between the first electrode and the secondelectrode and being comprised of a second phase change material. Thefirst phase change material has a first resistivity, the second phasechange material has a second resistivity, and wherein the firstresistivity is at least two times the second resistivity.

According to an embodiment of the present disclosure, a PCM cellcomprises a first electrode comprised of a first electrically conductivematerial, a second electrode comprised of a second electricallyconductive material, and a first phase change layer positioned betweenthe first electrode and the second electrode, the first phase changelayer having a first thickness and being comprised of a first phasechange material. The PCM cell further comprises a second phase changelayer positioned between the first electrode and the second electrode,the second phase change layer having a second thickness and beingcomprised of a second phase change material, and the second thickness isless than one-quarter of the first thickness.

According to an embodiment of the present disclosure, a method of usinga PCM cell comprising a first electrode, a second electrode, a dopedphase change layer positioned between the first and second electrodes,and a first undoped phase change layer in contact with the doped phasechange layer and one of the first and second electrodes is disclosed.The method comprises passing a first electrical current from the firstelectrode, through the first undoped phase change layer, to the secondelectrode to create a first amorphous zone in the doped phase changelayer that has an amorphous configuration, measuring a first electricalresistance between the first electrode and the second electrode throughthe first amorphous zone, and passing a second electrical current fromthe first electrode, through the undoped phase change layer, to thesecond electrode to anneal the first amorphous zone to have apolycrystalline configuration.

According to an embodiment of the present disclosure, a PCM cellcomprises a first electrode comprised of a first electrically conductivematerial, a second electrode comprised of a second electricallyconductive material, and an insulator comprised of an electricallyinsulating material positioned between the first and second electrodes.The PCM cell also comprises a first phase change layer positionedalongside the first and second electrodes and the insulator, the firstphase change layer being comprised of a mixture of a first phase changematerial and a dopant material, and the first phase change layer havinga first thickness, and a second phase change layer in contact with thefirst phase change layer and in an electrical path between the first andsecond electrodes, the second phase change layer consisting essentiallyof a second phase change material and having a second thickness. Thesecond thickness is less than half of the first thickness.

According to an embodiment of the present disclosure, a PCM cell, thePCM cell comprises a first electrode, a second electrode, and a pillarcomprised of a mixture of a first phase change material and a dopantmaterial, and the pillar having a first height. The PCM cell alsocomprises an insulator to surround the pillar, the insulator comprisingan electrically insulating material, and a layer consisting essentiallyof a second phase change material, the layer being in contact with thepillar and extending along an entirety of a side of the pillar, thelayer being in contact with at least one of the first and secondelectrodes, and the layer having a second height, wherein the secondheight is less than half of the first height.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-section view of a mushroom PCM cell with a dopedlayer in a polycrystalline configuration, in accordance with embodimentsof the present disclosure.

FIG. 1B is a cross-section view of the mushroom PCM cell with the dopedphase change material layer including an amorphized portion of the dopedphase change layer, in accordance with embodiments of the presentdisclosure.

FIG. 1C is a cross-section view of the mushroom PCM cell with the dopedphase change material layer including an amorphized portion of the dopedphase change layer and an undoped phase change material layer includingan amorphized portion of the undoped phase change layer, in accordancewith embodiments of the present disclosure.

FIGS. 2A-2F are cross-section views of alternate embodiment mushroom PCMcells, in accordance with embodiments of the present disclosure.

FIGS. 3A-3C are cross-section views of alternate embodiment bridge PCMcells, in accordance with embodiments of the present disclosure.

FIG. 4 is a cross-section view of alternate embodiment pillar PCM cell,in accordance with embodiments of the present disclosure.

FIGS. 5A-5G are cross-section views of alternate embodiment confined PCMcells, in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of the present disclosure. Itis noted that various connections and positional relationships (e.g.,over, below, adjacent, etc.) are set forth between elements in thefollowing description and in the drawings. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present disclosure is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layers “C”and “D”) are between layer “A” and layer “B” as long as the relevantcharacteristics and functionalities of layer “A” and layer “B” are notsubstantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus. Inaddition, any numerical ranges included herein are inclusive of theirboundaries unless explicitly stated otherwise.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements. It should benoted, the term “selective to,” such as, for example, “a first elementselective to a second element,” means that a first element can beetched, and the second element can act as an etch stop.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography.

Deposition can be any process that grows, coats, or otherwise transfersa material onto the wafer. Available technologies include physical vapordeposition (PVD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), molecular beam epitaxy (MBE) and more recently, atomiclayer deposition (ALD) among others. Another deposition technology isplasma enhanced chemical vapor deposition (PECVD), which is a processwhich uses the energy within the plasma to induce reactions at the wafersurface that would otherwise require higher temperatures associated withconventional CVD. Energetic ion bombardment during PECVD deposition canalso improve the film's electrical and mechanical properties.

Removal/etching can be any process that removes material from the wafer.Examples include etch processes (either wet or dry), chemical mechanicalplanarization (CMP), and the like. One example of a removal process ision beam etching (IBE). In general, IBE (or milling) refers to a dryplasma etch method which utilizes a remote broad beam ion/plasma sourceto remove substrate material by physical inert gas and/or chemicalreactive gas means. Like other dry plasma etch techniques, IBE hasbenefits such as etch rate, anisotropy, selectivity, uniformity, aspectratio, and minimization of substrate damage. Another example of a dryremoval process is reactive ion etching (ME). In general, ME useschemically reactive plasma to remove material deposited on wafers. WithRIE the plasma is generated under low pressure (vacuum) by anelectromagnetic field. High-energy ions from the ME plasma attack thewafer surface and react with it to remove material.

Semiconductor doping can be the modification of electrical properties bydoping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (“RTA”).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device.

Semiconductor lithography can be the formation of three-dimensionalrelief images or patterns on the semiconductor substrate for subsequenttransfer of the pattern to the substrate. In semiconductor lithography,the patterns are formed by a light sensitive polymer called aphoto-resist. To build the complex structures that make up a transistorand the many wires that connect the millions of transistors of acircuit, lithography and etch pattern transfer steps are repeatedmultiple times. Each pattern being printed on the wafer is aligned tothe previously formed patterns and gradually the conductors, insulatorsand selectively doped regions are built up to form the final device.

FIG. 1A is a cross-section view of PCM cell 100 with doped layer 102 ina polycrystalline configuration. FIG. 1B is a cross-section view of PCMcell 100 with doped layer 102 including zone 116 with an amorphousconfiguration. FIG. 1C is a cross-section view of PCM cell 100 withdoped layer 102 including zone 116 with an amorphous configuration andundoped layer 108-1 including zone 118 with an amorphous configuration.FIGS. 1A-1C will now be discussed in conjunction with one another.

In the illustrated embodiment, PCM cell 100 comprises heater 106,undoped layer 108-1 in contact with bottom electrode heater 106, dopedlayer 102 in contact with undoped layer 108-1, undoped layer 108-2 incontact with doped layer 102, and electrode 110 in contact with undopedlayer 108-2. The widths of undoped layer 108-1, doped layer 102, undopedlayer 108-2, and electrode 110 are the same, whereas the width of heater106 is substantially reduced, comparatively. Thereby, PCM cell 100 canbe said to have a mushroom configuration wherein an electrical currentcan flow from heater 106 to electrode 110 through undoped layer 108-1,doped layer 102, and undoped layer 108-2.

Heater 106 and electrode 110 can be comprised of an electricallyconductive material, such as metal, for example, titanium nitride (TiN).Heater 106 is an electrode that has a relatively narrow cross-sectionalarea, which focuses electrical current that is run through PCM cell 100.This allows heater 106 to generate heat through resistive heating, whichcan be used to change the temperature of PCM cell 100 (e.g., undopedlayer 108-1 and doped layer 102), for example, above the crystallizationtemperature and the melting temperature of doped layer 102. In addition,heater 106 can be comprised of multiple different electricallyconductive materials that can be arranged in multiple layers.

Undoped layers 108-1 and 108-2 (collectively, undoped layers 108) can becomposed essentially of a phase change material such as agermanium-antimony-tellurium (GST) gallium-antimony-tellurium (GaST), orsilver-iridium-antimony-telluride (AIST) material, although othermaterials can be used as appropriate. In addition, the phase changematerial of undoped layers 108-1 and 108-2 are the same in someembodiments, and they are different is other embodiments. The terms“composed essentially” and “consist essentially,” as used herein withrespect to materials of different layers, indicates that othermaterials, if present, do not materially alter the basic characteristicsof the recited materials. For example, a layer consisting essentially ofGST material does not include other materials that materially alter thebasic characteristics of the GST material. Hence, layers 108-1 and 108-2are also referred to as undoped layers. On the other hand, doped layer102 can be a mixture of phase change material 112 and phase separateddopant material 114. Doped layer 102 can also be a substitutional orinterstitial doped phase change material such as, for example,titanium-GST (TiGST), gallium-GST (GaGST), silicon-GST (SiGST), orbismuth-GST (BiGST), since these atoms can substitute/sit on intersticesdue to their solubility in GST. Phase change material 112 can be a GSTmaterial (although other materials can be used as appropriate which maybe the same as or different than undoped layers 108). Phase separateddopant material 114 can be one or more dielectric materials orpoorly-electrically conductive metals, such as SiO2, SiO, SiON, SiOC,carbon (C), tantalum nitride (Ta3N5), aluminum nitride (AlN), andtitanium nitride (TiN) although other materials can be used asappropriate. The grains of phase separated dopant material 114 canrestrict the grain size of phase change material 112 and provide “nanoopens” (i.e., local regions of relatively high electrical resistance) toincrease the resistance of doped layer 102, and, in some embodiments,the amount of phase separated dopant material 114 in doped layer 102 isat least 10% by volume.

In the illustrated embodiment, undoped layers 108 are substantiallythinner (in height) than doped layer 102. In addition, the thicknessesof undoped layers 108-1 and 108-2 are the same in some embodiments, andthey are different is other embodiments. In some embodiments, thethickness of undoped layers 108 is less than half of the thickness ofdoped layer 102. In some embodiments, the thickness of undoped layers108 is less than one eighth of the thickness of doped layer 102. In someembodiments, the thickness of undoped layers 108 is less than onesixteenth of the thickness of doped layer 102. In some embodiments, thethickness of undoped layers 108 is less than one eightieth of thethickness of doped layer 102. In some embodiments, the thickness ofundoped layers 108 is between 0.2 nanometers (nm) and 10 nm, and thethickness of doped layer 102 is between 20 nm and 100 nm. In someembodiments, the thickness of undoped layers 108 is between 0.5 nm and 5nm, and the thickness of doped layer 102 is between 40 nm and 80 nm.

In some embodiments, a cross-section of PCM cell 100 (into the page inFIGS. 1A and 1B) can be square, although in other embodiments, it can berectangular or circular. In some embodiments, the width or diameter ofheater 106 can be between 20 nm and 60 nm. In some embodiments, thewidth or diameter of heater 106 can be about 40 nm. In some embodiments,the width or diameter of doped layer 102, undoped layers 108, andelectrode 110 can be between 100 nm and 300 nm. In some embodiments, thewidth or diameter of doped layer 102, undoped layers 108, and electrode110 can be about 200 nm. In some embodiments, the width or diameter ofdoped layer 102, undoped layers 108, and electrode 110 can be greaterthan 200 nm.

In the illustrated embodiment, PCM cell 100 can be operated as a memorycell by passing an electrical current from heater 106 to electrode 110.This can be done at a variety of voltages to read or write a value onPCM cell 100. For example, to write, a high voltage can be used (e.g., 1volt (V) to 4 V) for a short period of time, which can cause heater 106to heat doped layer 102 beyond its melting point. Once the flow ofcurrent ceases, doped layer 102 can cool down rapidly, which forms zone116 in a process called “resetting”. Zone 116 is a dome-shaped region ofdoped layer 102 wherein phase change material 112 inside zone 116 is inan amorphous configuration, although the phase change material 112outside of zone 116 is still in a polycrystalline configuration. Inaddition, a similar “reset” zone 118 can be formed in undoped layer108-1, depending on the voltage, current, and/or duration of theresetting electrical pulse. Furthermore, the diameter of zone 118 can belarger than, the same as, or smaller than zone 116 (although it isdepicted as being larger in FIG. 1C). In general, this amorphousconfiguration has no definite structure. However, there can be local,disjoint crystalline nuclei (i.e., small crystallized regions of phasechange material 112) present in zone 116 and/or zone 118.

In some embodiments, zone 116 can be 40 nm tall and 40 nm in diameter.The creation of zone 116 can cause the electrical resistance across PCMcell 100 to increase as compared to a solely polycrystallineconfiguration of doped layer 102 (ala FIG. 1A). This new resistancevalue can then be read using current at a low voltage (e.g., 0.2 V)without changing the resistance value.

In addition, to again write PCM cell 100, phase change material 112 canbe returned back to a solely polycrystalline configuration by “setting”PCM cell 100. A high voltage can be used (e.g., 1 V to 4 V) for a shortperiod of time (e.g., 10 nanoseconds (ns)), which can cause heater 106to heat doped layer 102 beyond its crystallization point but not to itsmelting point. Since the crystallization temperature is lower than themelting temperature, once the flow of current ceases, doped layer 102can anneal and form crystals. This can cause the electrical resistanceacross PCM cell 100 to decrease as compared to having an amorphous zone116 (a la FIG. 1B) and/or zone 118 (a la FIG. 1C). This new resistancevalue can then be read using current at a low voltage (e.g., 0.2 V)without changing the resistance value.

In some embodiments, the melting temperature of undoped layers 108 anddoped layer 102 (i.e., the melting temperature of phase change material112 and phase change material 112 with phase separated dopant material114) can be about 600° C. In some embodiments, the crystallizationtemperature of doped layer 102 can be about 220° C., for example, ifphase separated dopant material 114 is SiO2. In some embodiments, thecrystallization temperature of undoped layers 108 can be about 180° C.In addition, the process of setting and resetting PCM cell 100 can occurrepeatedly, and in some embodiments, various zones 116 with differentresistances can be created in doped layer 102 (e.g., due to havingdifferent sizes and amounts of crystallization nuclei in zone 116). Thisallows for PCM cell 100 to be non-binary in that various distinctresistances can be created by varying the resetting parameters.

The ability to set and reset doped layer 102 can be facilitated byincluding undoped layers 108 between doped layer 102 and heater106/electrode 110. More specifically, phase separated dopant material114 in doped layer 102 can minimize the time and voltage conditions overwhich phase change material 112 can be electrically crystalized.Thereby, undoped layers 108 can function as transitions between the lowresistance of heater 106/electrode 110 and the high resistance of dopedlayer 102, allowing electrical current to spread over a larger area thanthat of the upper surface of heater 106. Undoped layers 108 can havebetter ohmic contact with heater 106/electrode 110 than doped layer 102would, and there is less variance in the low resistance states ofundoped layer 108-1 due to homogenous contact with heater 106. Inaddition, undoped layers 108 can homogenize the areas of ohmic contactwith doped layer 102 and heater 106/electrode 110, reducing the lowresistance state variance.

The components and configuration of PCM cell 100 provide analogcomputing capabilities. For example, the variable resistance of PCM cell100 can be set to correspond to values of analog weights. Instead ofmere 1's and 0's associated with a high and a low resistance statesuitable for binary computing, these weights can exist on a continuum ofanalog resistance values and can be used as artificial synapse weightsin a neural net for artificial intelligence computations and machinelearning. The relatively thin undoped layers 108 positioned betweenheater 106/electrode 110 and doped layer 102 widens the voltage rangethat can be used to set and reset PCM cell 100. This can be important ifdoped layer 102 has a high amount of doping because otherwise PCM cell100 may not be able to be set after being reset for the first time.Furthermore, the wider voltage range increases the range of operation ofthe memory programming function of PCM cell 100, which can allow for atighter distribution of resistance states leading to a more reliableoperation of PCM cell 100.

Depicted in FIGS. 1A-1C is one embodiment of the present disclosure, towhich there are alternatives. For example, doped layer 102 can bereplaced by another undoped layer that is as thick as doped layer 102.In such embodiments, the thick undoped layer can be a different materialthan the thin undoped layers 108 that has a substantially higherelectrical resistivity. In some embodiments, if undoped layers 108consist essentially of antimony telluride (Sb2Te3), then the thickundoped layer can consist essentially of GST225 (i.e., Ge2Sb2Te5). Whencrystalline, Sb2Te3 has a resistivity of about 0.0005 ohm*cm, whereasGST225 has a resistivity of about 0.003 ohm*cm. This is in contrast to adoped layer 102, which can have a resistivity of about 1 ohm*cm whencrystalline, which can be used with undoped layers 108 consistingessentially of GST225. While the resistivities of doped layer 102/thickundoped layer and undoped layers 108 can change with changingcrystalline or amorphous structure, generally, the resistivity of thethick undoped layer can be between 2 and 1000 times greater than theresistivity of undoped layers 108. In some embodiments, the resistivityof the thick undoped layer can be between 8 and 750 times greater thanthe resistivity of undoped layers 108, and in some embodiments, theresistivity of the thick undoped layer can be between 10 and 500 timesgreater than the resistivity of undoped layers 108.

For another example, one or both of undoped layers 108 can be replacedby another doped layer that is as thin as undoped layers 108,respectively. In such embodiments, the thin doped layer can be adifferent material than the thick doped layer 102 that has asubstantially lower electrical resistivity. Dopants that result in lowerresistivities can include, for example, titanium (Ti), bismuth (Bi),indium (In), and silver (Ag). These dopants are in contrast to dopantsthat result in higher resistivities such as, for example, SiO2, SiO,SiON, SiOC, C, Ta3N5, AlN, and TiN. Therefore, in some embodiments, ifdoped layer 102 comprises SiO2:GST, then thin doped layers can compriseTiGST.

FIGS. 2A-2F are cross-section views of alternate embodiment PCM cells200A-200E (collectively, PCM cells 200), respectively. PCM cells 200 canbe similar to or the same as PCM cell 100 (shown in FIG. 1 ) in certainaspects. Therefore, some of these aspects may have reference numeralsthat are one hundred greater than those of PCM cell 100.

In the illustrated embodiment of FIG. 2A, PCM cell 200-1 includes asingle, thin undoped layer 208-1 positioned between heater 206 and dopedlayer 202. In the illustrated embodiment of FIG. 2B, PCM cell 200-2includes a single, thin undoped layer 208-2 positioned between dopedlayer 202 and electrode 210. While PCM cells 200-1 and 200-2 have fewerundoped layers than PCM cell 100 has, the function and purpose ofundoped layers 208-1 and 208-2 can be the same as that of undoped layers108.

In the illustrated embodiment of FIG. 2C, PCM cell 200-3 includesprojection liner 218-1 positioned between heater 206 and undoped layer208-3, and undoped layer 208-3 is also in contact with doped layer 202.In the illustrated embodiment of FIG. 2D, PCM cell 200-4 includesprojection liner 218-2 positioned between heater 206 and doped layer202, and undoped layer 208-4 is positioned between doped layer 202 andelectrode 210. In the illustrated embodiment of FIG. 2E, PCM cell 200-5includes projection liner 218-3 positioned between heater 206 andundoped layer 208-5, and undoped layer 208-5 is also in contact withdoped layer 202. Furthermore, PCM cell 200-5 includes a second undopedlayer 208-6 positioned between doped layer 202 and electrode 210. In theillustrated embodiment of FIG. 2F, PCM cell 200-6 includes undoped layer208-7 positioned between heater 206 and projection liner 218-4, andprojection liner 218-4 is also in contact with doped layer 202.

Projection liners 218-1, 218-2, and 218-3 (collectively, projectionliners 218) can extend across the full width of undoped layer 208-3,doped layer 202, and undoped layer 208-5, respectively. Projectionliners 218 can be comprised of metal and/or semiconducting material.Thereby, projection liners 218 can provide a constant amount ofelectrical resistance and create a parallel current path around zone 116(shown in FIG. 1 ) when PCM cells 200-3, 200-4, and 200-5 are reset.

FIGS. 3A-3C are cross-section views of alternate embodiment PCM cells300-1, 300-2, and 300-3 (collectively, PCM cells 300), respectively. PCMcells 300 can be similar to or the same as PCM cells 100 (shown in FIGS.1A and 1B) or 200 (shown in FIGS. 2A-2E) in certain aspects. Therefore,some of these aspects may have reference numerals that are two hundredgreater than those of PCM cell 100 or one hundred greater than those ofPCM cells 200.

In the illustrated embodiment of FIG. 3A, PCM cell 300-1 includeselectrodes 306 and 310 with insulator 320 positioned in between andcoplanar with electrodes 306 and 310. Insulator 320 can be comprised ofan electrically insulating material such as, for example, alow-dielectric constant material, SiO2, silicon nitride (SiN), andfluorinated tetraethyl orthosilicate (FTEOS). In addition, undoped layer308-1 extends across the entire width of the top of electrode 306, andundoped layer 308-2 extends across the entire width of the top ofelectrode 310. Extending alongside the entire length of undoped layers308-1 and 308-2 (collectively undoped layers 308) and insulator 320 isdoped layer 302.

When current is run through PCM cell 300-1, it travels from electrode306 to electrode 310 through undoped layer 308-1, doped layer 302, andundoped layer 308-2. Therefore, PCM cell 300-1 has a bridgeconfiguration. Similar to PCM cell 100 (shown in FIGS. 1A and 1B),undoped layers 308 are thin compared to the thickness of doped layer302, which can have the same proportions and/or dimensions as those ofPCM cell 100. In some embodiments, doped layer 302 is between 5 nm and30 nm tall. In some embodiments, PCM cell 300-1 is between 100 nm and800 nm deep (i.e., the dimension going into the page in FIG. 2A).However, in some embodiments, doped layer 302 has a dog bone or bowtieshape that has a reduced depth of between 10 nm and 50 nm in the center.While PCM cell 300-1 has a different configuration of undoped layers 308than PCM cell 100 has, the function and purpose of undoped layers 308can be the same as that of undoped layers 108.

In the illustrated embodiment of FIG. 3B, PCM cell 300-2 includes asingle, thin undoped layer 308-3 that extends across the entire lengthof electrodes 306 and 310 and insulator 320. Thereby, doped layer 302extends along the entire length of undoped layer 308-3. When current isrun through PCM cell 300-2, it travels from electrode 306 to electrode310 though undoped layer 308-3 and doped layer 302 (whereby the currentthat travels though doped layer 302 will cross through undoped layer308-3 twice).

In the illustrated embodiment of FIG. 3C, PCM cell 300-3 includesprojection liner 318 that extends across the entire length of electrodes306 and 310 and insulator 320. Thereby, undoped layer 308-4 extendsalong the entire length of projection liner 318, and doped layer 302extends along the entire length of undoped layer 308-4. When current isrun through PCM cell 300-2, it travels from electrode 306 to electrode310 though projection liner 318, undoped layer 308-4, and doped layer302 (whereby the current that travels though undoped layer 308-4 willcross through projection liner 318 twice, and the current that travelsthough doped layer 302 will cross through undoped layer 308-4 andprojection liner 318 twice). While PCM cell 300-3 has a differentconfiguration of projection liner 318 than PCM cells 200-3, 200-4, and200-5 have, the function and purpose of projection liner 318 can be thesame as that of projection liners 218.

FIG. 4 is a cross-section view of alternate embodiment PCM cell 400. PCMcell 400 can be similar to or the same as PCM cells 100 (shown in FIGS.1A and 1B), 200 (shown in FIGS. 2A-2E), or 300 (shown in FIGS. 3A-3C) incertain aspects. Therefore, some of these aspects may have referencenumerals that are three hundred greater than those of PCM cell 100, twohundred greater than those of PCM cells 200, and one hundred greaterthan those of PCM cells 300.

In the illustrated embodiment, each of electrode 406, undoped layer408-1, doped layer 402, undoped layer 408-2, and electrode 410 has thesame diameter (e.g., about 40 nm). Therefore, PCM cell 400 has a pillarconfiguration. PCM cell 400 can be made by forming (e.g., depositing)each layer (i.e., electrode 406, undoped layer 408-1, doped layer 402,undoped layer 408-2, and electrode 410) and etching them into a pillar.

In addition, PCM cell 400 includes selector 422 beneath electrode 406.Selector 422 comprises carbon layer 424-1, ovonic threshold switch layer426 in contact with carbon layer 424-1, and carbon layer 424-2 incontact with ovonic threshold switch layer 426 and electrode 406.However, a person having ordinary skill in the art would recognize thatthere are other selectors that could be employed in PCM cell 400 besidesselector 422.

FIGS. 5A-5G are cross-section views of alternate embodiment PCM cells500-1, 500-2, 500-3, 500-4, 500-5, 500-6, and 500-7 (collectively, PCMcells 500). PCM cells 500 can be similar to or the same as PCM cells 100(shown in FIGS. 1A and 1B), 200 (shown in FIGS. 2A-2E), 300 (shown inFIGS. 3A-3C), or 400 (shown in FIG. 4 ) in certain aspects. Therefore,some of these aspects may have reference numerals that are four hundredgreater than those of PCM cell 100, three hundred greater than those ofPCM cells 200, two hundred greater than those of PCM cells 300, or onehundred greater than those of PCM cell 400.

In the illustrated embodiment of FIG. 5A, PCM cell 500-1 compriseselectrode 506, doped layer 502-1, undoped layer 508-1, electrode 510,and insulator 520. Doped layer 502-1 is substantially narrower thanelectrode 506 and undoped layer 508-1, so insulator 520 is positioned tooccupy the space around doped layer 502-1 within the projection ofelectrode 506 and undoped layer 508-1. Therefore, PCM cell 500-1 has aconfined pillar configuration.

Unlike PCM cell 400 (shown in FIG. 4 ), PCM cell 500-1 can be made byforming (e.g., depositing) electrode 506 and insulator 520. Then, a poreor via channel (i.e., a blind hole) can be formed (e.g., etched) ininsulator 520, and doped layer 502-1 can be formed (e.g., deposited andpolished) therein. Then, undoped layer 508-1 can be formed (e.g.,deposited) on top of doped layer 502-1 and insulator 520. Then,electrode 510 can be formed (e.g., deposited) on top of undoped layer508-1.

In the illustrated embodiment of FIG. 5B, PCM cell 500-2 includes ashorter undoped layer 508-2 compared to undoped layer 508-1. However,the function and purpose undoped layer 508-2 can be the same given thatundoped layer 508-2 extends across the entire top of doped layer 502-2.Since doped layer 502-2 is surrounded by insulator 520, current passedfrom electrode 506 to electrode 510 will still pass though undoped layer508-2.

In the illustrated embodiment of FIG. 5C, PCM cell 500-3 includes ashort undoped layer 508-3 at the bottom of doped layer 502-3. Therefore,PCM cell 500-3 can be made by forming (e.g., depositing) electrode 506and insulator 520. Then, a pore or via channel can be formed (e.g.,etched) in insulator 520, and undoped layer 508-3 can be formed (e.g.,deposited) therein. Then, doped layer 502-3 can be formed (e.g.,deposited and polished) on top of undoped layer 508-3. Then, electrode510 can be formed (e.g., deposited) on top of doped layer 502-3 andinsulator 520.

In the illustrated embodiment of FIG. 5D, PCM cell 500-4 includes a fullwidth undoped layer 508-4 underneath doped layer 502-4. Therefore, PCMcell 500-4 can be made by forming (e.g., depositing) electrode 506,undoped layer 508-4, and insulator 520. Then, a pore or via channel canbe formed (e.g., etched) in insulator 520 that extends down to undopedlayer 508-4. Then doped layer 502-4 can be formed (e.g., deposited andpolished) on top of undoped layer 508-4. Then, electrode 510 can beformed (e.g., deposited) on top of doped layer 502-4 and insulator 520.

In the illustrated embodiment of FIG. 5E, PCM cell 500-5 includes twoundoped layers 508-5 and 508-6 on either end of doped layer 502-5.Undoped layer 508-5 is only the width of doped layer 502-5, but undopedlayer 508-6 is the full width of electrode 510. In the illustratedembodiment of FIG. 5F, PCM cell 500-6 includes two undoped layers 508-7and 508-8 on either end of doped layer 502-6. Undoped layer 508-7 is thefull width of electrode 506, but undoped layer 508-8 is only the widthof doped layer 502-6. In the illustrated embodiment of FIG. 5G, PCM cell500-7 includes undoped layer 508-9 that extends along the sides andbottom of doped layer 502-6.

Further Discussion of Some Exemplary Embodiments

The following are non-exclusive descriptions of some exemplaryembodiments of the present disclosure.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode comprised of afirst electrically conductive material; a second electrode comprised ofa second electrically conductive material; a first phase change layerpositioned between the first electrode and the second electrode andbeing comprised of a first phase change material; and a second phasechange layer positioned between the first electrode and the secondelectrode and being comprised of a second phase change material; whereinthe first phase change material has a first resistivity; wherein thesecond phase change material has a second resistivity; and wherein thefirst resistivity is at least two times the second resistivity.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, further comprising: athird phase change layer in contact with the first phase change layerand the second electrode, the third phase change layer comprised of athird phase change material; wherein the third phase change material hasa third resistivity; and wherein the first resistivity is at least twotimes the third resistivity.

A further embodiment of any of the foregoing PCM cells, wherein: thethird phase change layer further comprises a dopant material.

A further embodiment of any of the foregoing PCM cells, wherein: thefirst phase change layer further comprises a dopant material.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer is composed of the second phase changematerial.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer is in contact with the first electrode and thesecond electrode.

A further embodiment of any of the foregoing PCM cells, furthercomprising: a projection liner positioned between the first electrodeand the second electrode and being comprised of a semiconductingmaterial.

A further embodiment of any of the foregoing PCM cells, wherein: theprojection liner is in contact with the first electrode and the secondphase change layer.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode comprised of afirst electrically conductive material; a second electrode comprised ofa second electrically conductive material; a first phase change layerpositioned between the first electrode and the second electrode, thefirst phase change layer having a first thickness and being comprised ofa first phase change material; and a second phase change layerpositioned between the first electrode and the second electrode, thesecond phase change layer having a second thickness and being comprisedof a second phase change material; wherein the second thickness is lessthan one-quarter of the first thickness.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, further comprising: athird phase change layer in contact with the first phase change layerand the second electrode, the third phase change layer having a thirdthickness and being comprised of a third phase change material; whereinthe third thickness is less than one-quarter of the first thickness.

A further embodiment of any of the foregoing PCM cells, wherein: thethird phase change layer further comprises a dopant material.

A further embodiment of any of the foregoing PCM cells, wherein: thefirst phase change layer further comprises a dopant material.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer is composed essentially of the second phasechange material that is undoped.

A further embodiment of any of the foregoing PCM cells, wherein: thefirst phase change layer has a first resistivity; the second phasechange layer has a second resistivity; and the first resistivity is atleast two times the second resistivity.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer is in contact with the first electrode and thesecond electrode.

A further embodiment of any of the foregoing PCM cells, furthercomprising: a projection liner positioned between the first electrodeand the second electrode and being comprised of a semiconductingmaterial.

A further embodiment of any of the foregoing PCM cells, wherein: theprojection liner is in contact with the first electrode and the secondphase change layer.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode comprised of afirst electrically conductive material; a second electrode comprised ofa second electrically conductive material; a first phase change layercomprised of a mixture of a first phase change material and a dopantmaterial, the first phase change layer having a first thickness; and asecond phase change layer in contact with the first phase change layerand only one of the first electrode and the second electrode, the secondphase change layer having a second thickness and being composedessentially of a second phase change material that is undoped; whereinthe second thickness is less than half of the first thickness.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, wherein: the secondphase change layer is in contact with the first phase change layer andthe first electrode; the PCM cell further comprises a third phase changelayer in contact with the first phase change layer and the secondelectrode, the third phase change layer having a third thickness andbeing comprised of a third phase change material that is undoped; andthe third thickness is less than half of the first thickness.

A further embodiment of any of the foregoing PCM cells, wherein thedopant material is a dielectric material.

A further embodiment of any of the foregoing PCM cells, wherein thedopant material is selected from a group consisting of: titanium,gallium, silicon, and nitrogen.

A further embodiment of any of the foregoing PCM cells, wherein thesecond phase change material consists essentially of germanium,antimony, and tellurium.

A further embodiment of any of the foregoing PCM cells, wherein thefirst electrode, the second electrode, the first phase change layer, andthe second phase change layer form a column with a constantcross-sectional area.

A further embodiment of any of the foregoing PCM cells, furthercomprising: a selector connected to the first electrode, the selectorcomprising an ovonic threshold switch.

A method of using a PCM cell, according to an exemplary embodiment ofthis disclosure, among other possible things, includes: a firstelectrode, a second electrode, a doped phase change layer positionedbetween the first and second electrodes, and a first undoped phasechange layer in contact with the doped phase change layer and one of thefirst and second electrodes, the method, according to an exemplaryembodiment of this disclosure, among other possible things, includes:passing a first electrical current from the first electrode, through thefirst undoped phase change layer, to the second electrode to create afirst amorphous zone in the doped phase change layer that has anamorphous configuration; measuring a first electrical resistance betweenthe first electrode and the second electrode through the first amorphouszone; and passing a second electrical current from the first electrode,through the undoped phase change layer, to the second electrode toanneal the first amorphous zone to have a polycrystalline configuration.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing method, further comprising:measuring a second electrical resistance between the first electrode andthe second electrode after annealing of the first amorphous zone;wherein the second electrical resistance is less than the firstelectrical resistance.

A further embodiment of any of the foregoing methods, furthercomprising: passing a third electrical current from the first electrode,through the first undoped phase change layer, to the second electrode tocreate a second amorphous zone in the doped phase change layer that hasan amorphous configuration.

A further embodiment of any of the foregoing methods, wherein the firstelectrical current passes through the first undoped phase change layerfirst and the doped phase change layer second.

A further embodiment of any of the foregoing methods, wherein: the PCMcell further comprises a second undoped phase change layer in contactwith the doped phase change layer; and the first electrical currentpasses through the first undoped phase change layer first, the dopedphase change layer second, and the second undoped phase change layerthird.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode comprised of afirst electrically conductive material and having a first width; asecond electrode comprised of a second electrically conductive materialand having a second width; a first phase change layer comprised of amixture of a first phase change material and a dopant material, thefirst phase change layer having a first thickness; and a second phasechange layer positioned between the first phase change layer and one ofthe first electrode and the second electrode, the second phase changelayer composed essentially of a second phase change material and havinga second thickness; wherein the first electrode is configured to heatthe first phase change layer to a temperature that is at least above acrystallization temperature of the first phase change material.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, wherein: the first widthis less than half of the second width; and the second thickness is lessthan half of the first thickness.

A further embodiment of any of the foregoing PCM cells, wherein thefirst thickness is between 20 nanometers (nm) and 100 nm and the secondthickness is between 0.2 nm and 10 nm.

A further embodiment of any of the foregoing PCM cells, wherein thefirst width is between 20 nm and 60 nm and the second width is between100 nm and 300 nm.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer is in contact with the first electrode; andthe first phase change material and the second phase change material arethe same and consist essentially of germanium, antimony, and tellurium.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode comprised of afirst electrically conductive material; a second electrode comprised ofa second electrically conductive material; an insulator comprised of anelectrically insulating material positioned between the first and secondelectrodes; a first phase change layer positioned alongside the firstand second electrodes and the insulator, the first phase change layerbeing comprised of a mixture of a first phase change material and adopant material, and the first phase change layer having a firstthickness; and a second phase change layer in contact with the firstphase change layer and in an electrical path between the first andsecond electrodes, the second phase change layer consisting essentiallyof a second phase change material and having a second thickness; whereinthe second thickness is less than half of the first thickness.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, wherein the firstthickness is between 20 nanometers (nm) and 100 nm and the secondthickness is between 0.2 nm and 10 nm.

A further embodiment of any of the foregoing PCM cells, wherein thefirst thickness is between 40 nm and 80 nm and the second thickness isbetween 0.5 nm and 5 nm.

A further embodiment of any of the foregoing PCM cells, wherein: thesecond phase change layer comprises a first portion that is separatedfrom the second portion; the first portion is in contact with andextends along the first electrode; the second portion is in contact withand extends along the second electrode; and the first phase change layeris in contact with and extends along the first portion, the insulator,and the second portion.

A further embodiment of any of the foregoing PCM cells, furthercomprising: a projection liner in contact with and extending along thefirst electrode, the insulator, and the second electrode, the projectionliner comprised of a material selected from a group consisting of: ametal material and a semiconductor material; wherein the second phasechange layer is in contact with and extends along an entirety of theprojection liner; and wherein the first phase change layer is in contactwith and extends along an entirety of the second phase change layer.

A PCM cell, according to an exemplary embodiment of this disclosure,among other possible things, includes: a first electrode; a secondelectrode; a pillar comprised of a mixture of a first phase changematerial and a dopant material, and the pillar having a first height; aninsulator to surround the pillar, the insulator comprising anelectrically insulating material; and a layer consisting essentially ofa second phase change material, the layer being in contact with thepillar and extending along an entirety of a side of the pillar, thelayer being in contact with at least one of the first and secondelectrodes, and the layer having a second height, wherein the secondheight is less than half of the first height.

The PCM cell of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations, and/or additional components:

A further embodiment of the foregoing PCM cell, wherein the first heightis between 20 nanometers (nm) and 100 nm, and the second height isbetween 0.2 nm and 10 nm.

A further embodiment of any of the foregoing PCM cells, wherein thelayer is in contact with the first and second electrodes.

A further embodiment of any of the foregoing PCM cells, furthercomprising: a selector connected to the first electrode, the selectorcomprising an ovonic threshold switch.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A phase change memory (PCM) cell comprising: afirst electrode comprised of a first electrically conductive material; asecond electrode comprised of a second electrically conductive material;a first phase change layer positioned between the first electrode andthe second electrode and being comprised of a first phase changematerial; a second phase change layer positioned between the firstelectrode and the second electrode and being comprised of a second phasechange material; and a projection liner positioned between the firstelectrode and the second electrode and being comprised of asemiconducting material; wherein the first phase change material has afirst resistivity; wherein the second phase change material has a secondresistivity; and wherein the first resistivity is at least two times thesecond resistivity.
 2. The PCM cell of claim 1, further comprising: athird phase change layer in contact with the first phase change layerand the second electrode, the third phase change layer comprised of athird phase change material; wherein the third phase change material hasa third resistivity; and wherein the first resistivity is at least twotimes the third resistivity.
 3. The PCM cell of claim 2, wherein thethird phase change layer further comprises a dopant material.
 4. The PCMcell of claim 1, wherein the first phase change layer further comprisesa dopant material.
 5. The PCM cell of claim 4, wherein the second phasechange layer is composed of the second phase change material.
 6. The PCMcell of claim 1, wherein the second phase change layer is in contactwith the first electrode and the second electrode.
 7. The PCM cell ofclaim 1, wherein the projection liner is in contact with the firstelectrode and the second phase change layer.
 8. A phase change memory(PCM) cell comprising: a first electrode comprised of a firstelectrically conductive material; a second electrode comprised of asecond electrically conductive material; a first phase change layerpositioned between the first electrode and the second electrode, thefirst phase change layer having a first thickness and being comprised ofa first phase change material; and a second phase change layerpositioned between the first electrode and the second electrode, thesecond phase change layer having a second thickness and being comprisedof a second phase change material; wherein the second thickness is lessthan one-quarter of the first thickness.
 9. The PCM cell of claim 8,further comprising: a third phase change layer in contact with the firstphase change layer and the second electrode, the third phase changelayer having a third thickness and being comprised of a third phasechange material; wherein the third thickness is less than one-quarter ofthe first thickness.
 10. The PCM cell of claim 8, wherein the thirdphase change layer further comprises a dopant material.
 11. The PCM cellof claim 8, wherein the first phase change layer further comprises adopant material.
 12. The PCM cell of claim 11, wherein the second phasechange layer is composed essentially of the second phase change materialthat is undoped.
 13. The PCM cell of claim 8, wherein: the first phasechange layer has a first resistivity; the second phase change layer hasa second resistivity; and the first resistivity is at least two timesthe second resistivity.
 14. The PCM cell of claim 8, wherein the secondphase change layer is in contact with the first electrode and the secondelectrode.
 15. The PCM cell of claim 8, further comprising: a projectionliner positioned between the first electrode and the second electrodeand being comprised of a semiconducting material.
 16. The PCM cell ofclaim 15, wherein the projection liner is in contact with the firstelectrode and the second phase change layer.
 17. A phase change memory(PCM) cell comprising: a first electrode comprised of a firstelectrically conductive material; a second electrode comprised of asecond electrically conductive material; an insulator comprised of anelectrically insulating material positioned between the first and secondelectrodes; a first phase change layer positioned alongside the firstand second electrodes and the insulator, the first phase change layerbeing comprised of a mixture of a first phase change material and adopant material, and the first phase change layer having a firstthickness; a second phase change layer in contact with the first phasechange layer and in an electrical path between the first and secondelectrodes, the second phase change layer consisting essentially of asecond phase change material and having a second thickness; and aprojection liner in contact with and extending along the firstelectrode, the insulator, and the second electrode, the projection linercomprised of a material selected from a group consisting of: a metalmaterial and a semiconductor material; wherein the second thickness isless than half of the first thickness.
 18. The PCM cell of claim 17,wherein: the second phase change layer comprises a first portion that isseparated from the second portion; the first portion is in contact withand extends along the first electrode; the second portion is in contactwith and extends along the second electrode; and the first phase changelayer is in contact with and extends along the first portion, theinsulator, and the second portion.
 19. The PCM cell of claim 17,wherein: the second phase change layer is in contact with and extendsalong an entirety of the projection liner; and the first phase changelayer is in contact with and extends along an entirety of the secondphase change layer.
 20. A phase change memory (PCM) cell comprising: afirst electrode comprised of a first electrically conductive material; asecond electrode comprised of a second electrically conductive material;a first phase change layer positioned between the first electrode andthe second electrode and being comprised of a first phase changematerial; a second phase change layer positioned between the firstelectrode and the second electrode and being comprised of a second phasechange material; and a heater positioned between the first electrode andthe second electrode and in contact with only one of the first phasechange layer and the second phase change layer; wherein the first phasechange material has a first resistivity; wherein the second phase changematerial has a second resistivity; and wherein the first resistivity isat least two times the second resistivity.
 21. The PCM cell of claim 20,wherein the PCM cell has a mushroom configuration such that a width ofthe heater is substantially reduced compared to a widths of the firstphase change layer and the second phase change layer.
 22. The PCM cellof claim 20, further comprising: a projection liner positioned betweenthe first electrode and the second electrode and being comprised of asemiconducting material.
 23. A phase change memory (PCM) cellcomprising: a first electrode comprised of a first electricallyconductive material; a second electrode comprised of a secondelectrically conductive material; a first phase change layer positionedbetween the first electrode and the second electrode, the first phasechange layer having a first thickness and being comprised of a firstphase change material; a second phase change layer positioned betweenthe first electrode and the second electrode, the second phase changelayer having a second thickness and being comprised of a second phasechange material; and a heater positioned between the first electrode andthe second electrode and in contact with only one of the first phasechange layer and the second phase change layer; wherein the secondthickness is less than one-quarter of the first thickness.
 24. The PCMcell of claim 23, wherein the PCM cell has a mushroom configuration suchthat a width of the heater is substantially reduced compared to a widthsof the first phase change layer and the second phase change layer. 25.The PCM cell of claim 23, further comprising: a projection linerpositioned between the first electrode and the second electrode andbeing comprised of a semiconducting material.